Again SQL statements have been replaced with functions from acct_mgr.model,
some other code corrections and simplifications done, quoting style changes
especially for log messages accompanied by a complete revision of Python doc
strings and code comments.

If ignore_auth_case is True, username conversion to lowercase is required.
I guess, that adding a corresponding hint to the registration page will avoid
confusion about this conversion of all user input for the username field.

The longer I thought about it, the more I've been convinced, that a username
differing only by character case is spelling trouble for any environment.
I remembered having such problems when switching ignore_auth_case to True
for a Trac instance some time ago and couldn't think of a use-case anyway,
so I made case-less checks mandatory.

Even better, pulling the obsoleted new option now is fighting feature bloat:
With 25+ options AccountManager is not exactly easy to configure, so every
dispensed option is a blessing.

Additionally I hope, that a highlighted username improves error messages.